The present invention relates to methods and compositions for treating subterranean formations. More particularly, the present invention relates to treatment fluids that comprise a methyl ester sulfonate (“MES”) surfactant, and associated acidizing methods.
The production of desirable fluids (e.g., oil and gas) from subterranean formations may often be enhanced by stimulating a region of the formation surrounding a well bore. Where the subterranean formation comprises acid-soluble components, such as those present in carbonate and sandstone formations, stimulation is often achieved by contacting the formation with a treatment fluid comprising an acid. These acid stimulation treatments are often referred to as “acidizing” the formation. For example, where hydrochloric acid contacts and reacts with calcium carbonate in a formation, the calcium carbonate is consumed to produce water, carbon dioxide, and calcium chloride. After acidization is completed, the water and salts dissolved therein may be recovered by producing them to the surface, e.g., “flowing back” the well, leaving a desirable amount of voids (e.g., wormholes) within the formation, which enhance the formation's permeability and may increase the rate at which hydrocarbons subsequently may be produced from the formation. One method of acidizing, known as “fracture acidizing,” comprises injecting a treatment fluid comprising an acid into the formation at a pressure sufficient to create or enhance one or more fractures within the subterranean formation. Another method of acidizing, known as “matrix acidizing,” comprises injecting the treatment fluid into the formation at a pressure below that which would create or enhance one or more fractures within the subterranean formation.
To enhance acidizing treatments, various additives may be added to the treatment fluid. One such additive is a gelling agent which may, among other things, increase viscosity of the treatment fluid for improved diversion and particulate suspension, increase penetration into the reservoir by decreasing the reactivity of such fluid, reduce fluid loss, and/or reduce pumping requirements by reducing friction in the well bore. In some instances, the acidizing treatment may be self-diverting to further enhance the treatment. Among other things, a self-diverting acid treatment may effectively place the acid in a desired region within the subterranean formation, thereby creating a more optimal interaction of the acid with the acid-soluble components of the formation, which may create a desired network of channels that may penetrate deeper into the formation than a conventional acid stimulation treatment. One such self-diverting treatment fluid includes a crosslinkable gelling agent, a crosslinking agent, and a pH buffer to provide a crosslink within a certain pH range. A crosslinkable gelling agent comprising crosslinkable polyacrylamide-based polymers has been found to be useful in calcium carbonate formations. In such a treatment, as the acid reacts, the pH of the self-diverting treatment fluid increases, which causes the fluid to viscosify so as to form a gel that, inter alia, temporarily plugs the perforations or natural fractures accepting the most fluid flow. When the remaining treatment fluid encounters the gel, it is diverted to other portions of the formation. This process then may be repeated—as the treatment fluid is diverted, the acid creates another conductive void, and the treatment fluid is viscosified, diverts flow, and so forth. Once the treatment is complete, the viscosified treatment fluid may be “broken” by reducing its viscosity to a more readily pumpable level, so that the full productivity of the well can be restored.
Despite the advantages of using gelling agents in acid treatments, such treatments may be problematic. For example, conventional polymeric gelling agents may leave an undesirable residue in the subterranean formation after use. As a result, potentially-costly remedial operations may be required to clean up the surfaces inside the subterranean formation. Foamed treatment fluids and emulsion-based treatment fluids have been employed to minimize residual damage, but increased expense and complexity often result.
To combat these problems associated with polymeric gelling agents, some surfactants have been used as gelling agents. Certain surfactants, when mixed with an aqueous fluid having a certain ionic strength, are capable of forming a viscous fluid that has certain elastic properties, one of which may be shear thinning. Surfactant molecules (or ions) at specific conditions may form micelles (e.g., worm-shaped micelles, rod-shaped micelles, etc.) in an aqueous fluid. Depending on, among other things, the surfactant concentration, and the ionic strength of the fluid, etc., these micelles may impart increased viscosity to the aqueous fluid, such that the fluid exhibits viscoelastic behavior due, at least in part, to the association of the surfactant molecules contained therein.
Accordingly, these treatment fluids exhibiting viscoelastic behavior may be used in a variety of subterranean treatments where a viscosified treatment fluid may be useful. For instance, these surfactants may be used in place of conventional polymeric gelling agents in acidizing treatments. In some instances, as the acid reacts with the formation, the reaction by-products and/or spending of the acid may provide the conditions necessary for viscosification of the treatment fluid so as to form a gel that, inter alia, temporarily plugs the perforations or natural fractures accepting the most fluid flow and diverts the remaining treatment fluid and/or another fluid to other regions of the formation. Because the micelles are sensitive to the pH and hydrocarbons, once viscosified, the viscosity of the treatment fluid may be reduced after introduction into the subterranean formation without the need for conventional gel breakers (e.g., oxidizers). This should allow a substantial portion of the treatment fluid to be produced back from the formation without the need for expensive remedial treatments. However, surfactants used heretofore as gelling agents tend to have undesirable environmental characteristics (e.g., toxicity) and/or may be limited by strict environmental regulations in certain areas of the world.